The star won the prize for her role as a ballet dancer in Black Swan.
Wearing a stunning white gown that hung loose and covered any possible baby bump, Portman praised director Darren Aronofsky for the movie - and for her amazing news in her personal life.
'I keep telling Darren - you made me very skinny, and now you're indirectly responsible for getting me fat because you introduced me to my love, Benjamin Millepied, who also choreographed the film. And I am incredibly grateful for that most, most special of gifts,' she said.
She also paid tribute to her parents: 'If I am a shadow as good as you are as parents - an echo of how good you are as parents - I will be the best, so thank you.'